This invention relates to intrusion detectors, and particularly to a sensor for use in a leaky cable intrusion detector system.
Leaky cable intrusion detector systems use a pair of transmission cables which leaks electromagnetic radiation. One cable is connected to a transmitter and the other to a receiver. The transmitter applies either a pulsed radio frequency signal or a continuous wave signal to one cable, and the leaked radiation sets up a field which is coupled to the second cable. The resulting signal is detected in the receiver. In the case of an intruder passing into the field, a phase shift or time change of the received signal occurs with respect to transmitted signal, which allows determination of the existence or the location of the intrusion. Such systems are described in the paper by Dr. R. Keith Harman and John E. Siedlarz given to the 1982 Carnahan Conference on Security Technology, at the University of Kentucky, May 12-14, 1982. It should be noted that in some systems the transmitter can be located at one end of one cable, and the receiver at the adjacent end of the other cable, and in other systems the receiver can be located at the remote end of the second cable.
One form of pulsed radio frequency signal leaky cable intrusion detector system is described in Canadian Patent No. 1,014,245 issued July 19th, 1977, invented by Dr. R. Keith Harman. Another system, using continuous wave signals is described in Canadian patent application No. 403,015 filed May 14th, 1982, invented by Dr. R. Keith Harman and Dale R. Younge.
It has been found that a sensitivity problem exists with such systems which use buried parallel leaky coaxial cables as a sensor. It has been found that due to variations in the characteristics of the burial medium (e.g. the earth), the cables exhibit variations in sensitivity along their length. The sensitivity can vary within a relatively short distance from excessive, through normal, to inadequate. In regions of inadequate sensitivity, the presence of an intruder may not be detected. In regions of excessive sensitivity, the presence of a passing automobile or other objects far outside the normal detection range can cause the system to set off an alarm. Such variations in sensitivity are clearly undesirable, and can be fatal to the reliable operation of the system.
The present invention is directed to a sensor for such systems which can substantially reduce the excessive variations in sensitivity.
Further, the use of the present invention creates a detection field which has varying height, in cross-section, which has been found to reduce the possibility of a person passing through the detection field by attempting to remain below the detection threshold, by reducing his body size, i.e. by crouching or "duck walking".
In addition, the present invention makes feasible a leaky cable intrusion system in which the transmitter and receiver are coupled to opposite ends of the corresponding cables, and in which the cables are not graded, yet allows determination of the location of an intrusion along the cables. This is made possible with the use of cables having different velocity of propagation characteristics.